As a method for imparting water/oil repellency to the surface of a porous substrate such as fabric, a method of treating the substrate with a water/oil repellent composition composed of an emulsion having, dispersed in an aqueous medium, a copolymer having units based on a monomer having a polyfluoroalkyl group (hereinafter a polyfluoroalkyl group will be referred to as a Rf group) having at least 8 carbon atoms has been known.
However, recently, EPA (US Environmental Protection Agency) has pointed out that a compound having a perfluoroalkyl group (hereinafter a perfluoroalkyl group will be referred to as a RF group) having at least 7 carbon atoms is likely to be decomposed in the environment or in the body, and the decomposition product is likely to be accumulated, i.e. it presents a high environmental impact. Therefore, a water/oil repellent composition comprising a copolymer which has units based on a monomer having a RF group having at most 6 carbon atoms, and has no units based on a monomer having a RF group having at least 7 carbon atoms, has been required.
As a water/oil repellent composition comprising such a copolymer, for example, the following water/oil repellent compositions (1) and (2) have been proposed.
(1) A water/oil repellent composition comprising a copolymer having units based on the following monomer (a), units based on the following monomer (b) and units based on the following monomer (c), and a liquid medium, wherein the molar ratio ((a)/(b)) of the units based on the monomer (a) to the units based on the monomer (b) is at least 1 (Patent Document 1).
Monomer (a): vinyl chloride.
Monomer (b): a vinyl ether having no Rf group and having a crosslinkable functional group.
Monomer (c): a monomer having a RF group having at most 6 carbon atoms.
(2) A water/oil repellent composition comprising a copolymer having units based on the following monomer (a) and units based on the following monomer (b), and as the case requires, units based on the following monomer (c), and having a mass average molecular weight of at least 40,000, and a liquid medium (Patent Document 2).
Monomer (a): a monomer having a RF group having at most 6 carbon atoms.
Monomer (b): vinylidene chloride.
Monomer (c): a monomer having no Rf group and having a crosslinkable functional group.
By the way, in an article having its surface treated with a water/oil repellent composition (a textile product such as a sports wear), a moisture-permeable waterproofing film may be formed on the back side so as to impart a function to release water vapor by sweating from the body and a function to prevent penetration of rain (Patent Document 3).
As a method for producing fabric having such a moisture-permeable waterproofing film, a coating method is, for example, well known.
The coating method is a method of applying a coating liquid containing a polyurethane resin, an acrylic resin or the like to the back side of fabric treated with a water/oil repellent composition. For the fabric, it is required that the bonded or applied resin film is not easily peeled off.
As a liquid medium contained in the coating liquid, in the case of dry coating of forming a resin film by drying after application, a highly volatile solvent such as methyl ethyl ketone, ethyl acetate, toluene or isopropyl alcohol is used. Whereas, in the case of wet coating of forming a resin film by solidification in water, a polar organic solvent such as N,N-dimethylformamide (hereinafter referred to as DMF) is used.